White mould is a fungal disease that infects many crops, including canola. The causative agent, Sclerotinia sclerotiorum, occurs in all canola-growing regions of Canada and throughout the world, causing stem rot of the plants. Annual losses due to this fungus are highly variable, ranging from 5 to 100%; in 2010, 90% of Canadian canola crops showed some level of Sclerotinia infection and the loss to growers was estimated at $600 million. With no available Sclerotinia-resistant cultivars available, damage from this fungus is best controlled by crop rotations and foliar fungicides. Unfortunately, under damp climatic conditions, such methods are insufficient to limit the impact of the disease. In addition, there is increasing public concern over the risk that chemicals pose to the environment and human health. Together, these present compelling reasons to find safe (fungal or species-specific) alternatives to control this costly fungal pathogen.

One technology that offers the promise of species-specific control of crop pests and pathogens is RNA interference (RNAi). RNAi is a method of reducing a targeted gene’s expression through the application of double-stranded RNA (dsRNA). As every species is defined by the uniqueness of their gene sequences, dsRNAs can be designed to be species-specific. We recently explored the ability of RNAi to inhibit the expression of key genes in Sclerotinia, with the aim to identify dsRNAs that can reduce or prevent the growth of the fungus. Over 60 different dsRNA molecules were tested, and 20 of them strongly inhibited the growth of the fungus. By mixing dsRNAs, we could target different biochemical pathways within Sclerotinia, and thereby enhanced the impact on the fungus. Using the most potent of these dsRNAs, we have developed two alternative approaches that protected canola plants: a transgenic technology, where plants produced the protective dsRNAs continuously; and a foliar spray that adhered to the leaves and prevented the spread of the infection throughout the plant.

Both of these technologies provide a new generation of fungicides to protect canola against Sclerotinia. Transgenic technology offers the advantage of providing full protection to the plants, but requires considerable time to develop, test, and to acquire regulatory approval. Foliar dsRNA sprays, in contrast, will require less time to develop and test on any crops that are prone to Sclerotinia infections. As they are a new class of pesticides, dsRNAs will still need to be reviewed with some scrutiny, but their built-in specificity is a distinct advantage over broad-spectrum pesticides, which should be viewed more favourably by the public and regulators.

High-throughput dsRNA synthesis methods are now available, such that a single plant can be treated with dsRNA for a fraction of a cent. With continued investment in this technology, the cost will continue to drop, and could provide canola growers with safe and affordable alternatives to our current conventional chemistries to prevent Sclerotinia outbreaks.

This research was co-sponsored by Manitoba Agriculture’s Growing Innovation: Agri-Food Research and Development Initiative and the Western Grains Research Foundation.